The Nervous System

Everything you notice, decide and do travels the same route: something detects a change, a signal runs inwards, somewhere in the middle a decision gets made, and a signal runs back out to a muscle. Learn that route once and most of this topic falls into place.

The two halves of the system

The split is simple and worth getting right, because questions often hinge on it.

• CNS — brain and spinal cord. This is where information is processed.
• PNS — every nerve outside those two. This is the wiring that reaches the sense organs, muscles and glands.

Impulses are electrical signals passing along nerve cells. Nerves themselves are bundles of many neurones wrapped together, rather like a cable containing many separate wires. Some neurones are myelinated — wrapped in a fatty sheath by Schwann cells, with small gaps called nodes of Ranvier — which makes impulses travel much faster.

The brain, region by region

The brain is billions of interconnected neurones, and different regions have taken on different jobs. You do not need the fine detail — you need to be able to name a region and say what it does.

The spinal cord is not just a cable

It is tempting to think of the spinal cord as a wire that carries messages up to the brain. It does do that — but it also makes decisions itself, which is why the syllabus calls it an integration centre.

Cut across the cord and you see two kinds of tissue:

• White matter — mostly the myelinated axons of neurones carrying information up to and down from the brain. It looks white because of all the fatty myelin.
• Grey matter — the cell bodies and synapses. This is where signals are actually processed, and where relay neurones live.

When information enters along a sensory neurone and leaves along a motor neurone without ever going to the brain, you get a reflex. That is unconscious control, directed by the spinal cord alone.

Input: receptors and sensory neurones

Every neural pathway starts with a receptor — a specialised cell that detects a change in the environment. A change that a receptor can detect is called a stimulus.

That conversion is why receptors are described as transducers: they take energy in one form and turn it into electrical energy in a sensory neurone. Some receptors are separate specialised cells that pass their signal on to a neurone; others are simply the bare endings of the sensory neurone itself, which is common for touch.

All or nothing: the threshold

When a receptor cell is stimulated it becomes depolarised — the inside becomes less negative. But depolarising a little bit is not enough.

• Weak stimulus → the cell is not depolarised enough → no impulse is sent at all.
• Strong enough stimulus → an action potential is started in the sensory neurone → the impulse travels to the CNS.

This is genuinely useful. It filters out the constant background noise of tiny changes, so your brain only hears about things that matter.

Worked through: how a salty crisp becomes an impulse

The tongue is covered in small bumps called papillae, each covered in taste buds, each containing chemoreceptor cells covered in receptor proteins. Different receptor proteins detect different chemicals.

Output: motor neurones and muscles

Once the CNS has decided, motor neurones carry action potentials out to the effectors. Conscious movements are worked out by the motor cortex, a region of the cerebrum.

A motor neurone does not touch the muscle. It ends at a neuromuscular junction (also called a motor end plate), which works in almost exactly the same way as a synapse. One muscle contains many neuromuscular junctions spread across its fibres.

Once calcium ions are in the sarcoplasm they bind to troponin molecules and make them change shape. Troponin and tropomyosin shift position on the thin actin filaments, which uncovers the myosin-binding sites. Myosin can now grab actin, and the sliding filament model of contraction begins.

Worked examples